In recent years it has become increasingly important for environmental and other reasons for operators of underground storage tanks (USTs) to monitor for leakage of the product contained therein. This is particularly important for operators of gasoline filling stations, which use many USTs, as these users have come under increasing environmental scrutiny. The United States Environmental Protection Agency (EPA) has issued strict regulations as to procedures which must be adopted by UST operators. Furthermore, there is a strong possibility of civil liability for an owner of a UST which has leaked gasoline or other chemicals into the soil and into the water table.
The EPA has approved a number of leak detection methods that can be used by UST owners. These arc monthly monitoring options, including round water monitoring, automatic tank gauging (ATG), vapor monitoring, secondary containment with interstitial monitoring, and statistical inventory reconciliation (SIR). In addition, a UST owner can use monthly inventory control with annual tank tightness testing. Regulations in several states have limited these choices further. While each of these methods has its strengths, all of them have shown many weaknesses including false alarms, requirements for tank downtime, and monitoring that is not continuous. Furthermore, these methods do not provide means to continuously check gasoline dispenser meter calibration, and a miscalibration can lead to inaccurate reports and, more importantly, can mask an otherwise detectable leak.
Historically, leak detection for a UST was performed through manual inventory control. The operator would determine the volume contained in the tank with a graduated stick, and reconcile the physical inventory with the book inventory calculated from sales and deliveries. Manual inventory control, however, proved inadequate as a leak detection system for various reasons; the minimum detectable daily leak volume depends on the accuracy of stick measurement performed by the operator. Means approved by the EPA were established to improve upon manual inventory control. The methods approved to date by the EPA use one of three basic approaches: methods to eliminate the dynamics of the process, methods that attempt to account for the dynamics of the process, and methods that look for evidence of a leak by using sensors external to the tank.
Using the basic assumption that leaks can be detected by eliminating the process dynamics, ATGs have proven to be one of the most popular methods of detection. These devices have been described in the prior art and detect leaks when they are used in a leak detection mode that requires a dormant tank. A tank-specific device for detecting leaks is shown in U.S. Pat. No. 4,972,710 to Uhlarik, et al.
A variety of techniques are used in ATGs to measure temperature and product level changes over the length of the test. Since the tank is required to be dormant for the test, any drop in tank level is assumed to be evidence of a leak. By having sufficient measurement precision for this once-a-month test, these devices can meet the EPA leak rate requirements with a two to eight hour test period. While the test time has been decreased by increased precision, the disruption of normal tank activity still causes a problem. Even a two to eight hour period once per month can be difficult to schedule in today's continuous petroleum retail operations. The net result is that the test may not be done at the 30 day schedule as required. In addition, while the 30 day cycle presents the above-mentioned operational problems, it is understandable that a leak occurring in midpoint between monthly tests could do a significant amount of damage before it is detected at the next test. Thus, it would be preferable from an environmental protection standpoint to test more often so that leaks can be detected as soon as possible.
In addition, the trade-off of decreasing measurement time by increasing precision has resulted in an increased susceptibility to secondary effects and has led to false alarms. UST owners must respond to these false alarms or face future liability problems if an actual loss occurs. Even worse, recent data have indicated that only a small percentage of installed ATGs are functioning correctly and are, in fact, giving misleading readings and reports to tank owners, leading them to have a false sense of security.
Furthermore, most devices described in the prior art measure product volume by using a sensor to determine the product height. The product level is then converted to volume by the device through a formula or table determined by the physical construction and placement of the tank. It has been found that this conversion process is frequently in error due to one or more possible problems. While the tanks are made to standard sizes, steel tanks, for example, are permitted to have a five percent (5%) volumetric variation, so the exact size of the tank is not known. This variation can cause large discrepancies in calculating the product volume based upon the measured level. An even larger error can occur when the device is misprogrammed due to inaccurate knowledge of the actual tank dimensions or through human error. Additionally, the tank may be placed in the ground tilted or it may tilt over time causing a change in the level to volume relationship. Furthermore, the tank may deform in the ground over time causing a change in the level to volume relationship. Still further, some devices use approximation methods to determine the level to volume relationship that are inherently inaccurate for certain sections of a cylindrical tank.
Another method of leak detection is known as statistical inventory reconciliation (SIR). Based on historical inventory control, SIR methods use measurements from an active system and attempt to understand the dynamic influences on that system. As compared to the ATG, increased test length is traded for the precision of each measurement. The basic technique of SIR involves using statistical analysis to analyze the difference noted when the daily physical inventory change is compared to the difference between product sold and delivered. The UST operator records sales, delivery and inventory information each day and the analysis is usually performed once every month on the last 30 days of data.
However, the dynamics of the active system under measurement may induce a multitude of errors in the collected data under this process. These errors could include improper measurement of the product level, especially if the tank level is determined manually through the use of a "stick"; errors in converting the product level to volume; data entry errors; miscalibration of dispenser meters leading to inaccurate sales information; over and under delivery of product; pilferage or other unrecorded removals of product; losses due to line leakages; and losses due to tank leakages. Current analysis algorithms must focus on identification and discrimination of these multiple effects. But because the analysis is retrospective and based on data that is essentially a series of daily gross measurements, identification of these multiple effects is difficult to confirm. As with the ATG, the 30 day test is not completely satisfactory from an environmental protection viewpoint, and a more timely test (each day, for example) would minimize losses.
For SIR methods to be effective, accurate data and consistent collection are necessary. To help, an ATG can be used to provide accurate level information, eliminating the graduated stick, while point of sale ("POS") computer systems, which generally consist of personal computers known in the prior art, can provide an electronic aid to the collection of sale and delivery information.